The Republican party’s latest attempt to overhaul US healthcare ran into trouble on Thursday when at least four Republican senators said they would not support a newly unveiled Senate reform bill, casting fresh doubt over Donald Trump’s ability to deliver on a key campaign promise.

Republicans need the votes of at least 50 of their 52 senators for the bill to pass. But by Thursday afternoon, Ted Cruz, Rand Paul, Mike Lee and Ron Johnson, all members of the party’s conservative faction, had announced they would not cast a vote to support the bill in its current form.

Mr Paul, the conservative senator from Kentucky, said: “The current bill does not repeal Obamacare. It does not keep our promises to the American people.”

Mr Trump, who has watched healthcare turn into a quagmire for his administration, suggested there remained room to revise the bill to win over sceptics. “A little negotiation, but it’s going to be very good,” he said earlier in the day.

Later in the evening, Mr Trump offered a more full-throated endorsement of the legislation. “I am very supportive of the Senate #HealthcareBill. Look forward to making it really special!” the president wrote on Twitter.

After weeks of secret talks, Mitch McConnell, the Republican Senate majority leader, had released a bill that sought to bridge divisions between conservative senators who want to limit the role of government in healthcare and moderate Republicans who are concerned about people losing insurance.

The legislation would end a requirement for Americans to have health insurance, cut the Medicaid programme for the poor, and repeal taxes on the rich.

The bill was more moderate than an original version passed by the House of Representatives in May, stopping short of Mr Trump’s pledge to dismantle his predecessor’s healthcare system and instead maintaining some of its basic architecture.

If the bill passed the Senate, it would take Mr Trump a step closer to scoring the first big legislative achievement of his presidency after months of mis-steps and setbacks.

From the party’s moderate flank, Senator Dean Heller, a Republican from Nevada who is up for re-election in 2018, said: “At first glance, I have serious concerns about the bill’s impact on the Nevadans who depend on Medicaid.”

A spokeswoman for Susan Collins, a Republican from Maine, said the moderate senator was waiting to see the Congressional Budget Office’s independent assessment of the bill’s financial impact. The spokeswoman said Ms Collins had “a number of concerns” about the bill’s effects on insurance premiums and the Medicaid programme.

Pat Roberts, a Republican senator from Kansas, told reporters: “This is not the best possible bill. This is the best bill possible under very difficult circumstances.” He said the current bill would “take some time to fix” but said it was a necessary stepping stone towards saving the current healthcare system and “getting off the cliff”.

The current bill does not repeal Obamacare. It does not keep our promises to the American people

“What’s the other alternative? I don’t see anything from the other side,” he said.

Barack Obama also weighed in on the legislation, arguing that the current plan to dismantle his signature healthcare programme would disastrously impact millions of Americans, particularly young people and women.

“We didn’t fight for the Affordable Care Act for more than a year in the public square for any personal or political gain — we fought for it because we knew it would save lives, prevent financial misery, and ultimately set this country we love on a better, healthier course,” the former president wrote on Facebook.

“The Senate bill, unveiled today, is not a healthcare bill. It’s a massive transfer of wealth from middle-class and poor families to the richest people in America. It hands enormous tax cuts to the rich and to the drug and insurance industries, paid for by cutting healthcare for everybody else,” he said.

One Republican lawmaker who backs the bill said even Republican senators who have concerns about some elements would find it difficult to oppose the bill given the overwhelming desire among the Republican grassroots to repeal Obamacare.

The lawmaker said Republican leaders would rather avoid a “conference” — the process where the House and Senate try to forge a compromise bill — because it would be messy and increase the likelihood of new controversial elements being included. As a result, he said the House was likely to come under pressure to vote on the Senate bill.

Brandon Barford, a former Senate aide and partner at Beacon Policy Advisors, said Mr McConnell had worked hard to win as much support as possible. “He’s basically tried to create something for everyone to get something they want. So no one feels like they totally lost, but no one feels like they won entirely,” he said.

The bill would repeal most of the tax increases that were imposed on the wealthy as part of Obamacare to pay for the reforms. It pays for the repeal of those taxes by shrinking the Medicaid programme for low-income Americans.

Republicans are gearing up for aggressive attacks from Democrats who have latched on to the fact that Mr Trump told a group of GOP senators that he thought the original House bill was “mean”. His quip angered Republicans by handing ammunition to Democrats and came despite the fact that he had held an event to celebrate the House bill.

On Thursday, Democratic protesters rallied outside Mitch McConnell’s office, where some of them were dragged away by police. On the Senate floor, senators traded boos and barbs, with Democrats criticising Republicans for pushing out a bill behind closed doors, and Republicans accusing Democrats of criticising a piece of legislation that they had not yet had the time to read.

“[Senate Democrats] can choose to keep standing by as their failing law continues to collapse and hurt more Americans, but I hope they will join with us instead to bring relief to the families who have struggled under Obamacare for far too long,” Mr McConnell said.

Senator Chuck Schumer, the top Democrat, countered that Republicans were embarrassed about the bill, noting that if they were not they would have “brass bands” touting its release in every city in America.